In oil and gas producing areas, government regulations for the abandonment of bore holes require that the different strata of bore holes be permanently isolated by the placement of cement plugs at critical locations in the bore hole. The purpose of the plugs is to prevent the migration of fluids and gases from one strata to another, thereby avoiding contamination of the water table, the escape of hydrocarbons to the atmosphere, etc.
An open hole reference base log is recorded as part of the `suite` of logs which are recorded when the drilling of a well is completed. The reference base log is a graphic record of the natural gamma radiation of the formations through which the bore hole passes. It provides information about the composition and the location of the strata traversed and serves as a map which is used to determine the number and location of abandonment plugs, as required by government regulations. Abandonment plugs must be positioned in the bore hole in accordance with the depth measurements which are established when the open hole reference base log is created, as this log serves as the standard depth reference for the lifetime of a well.
A typical procedure for the placement of cement plugs in an abandoned bore hole is as follows:
An abandoned bore hole is usually filled with drilling mud, a mixture of water and small sized drilling debris particles, before cement abandonment plugs are placed. A drill pipe is lowered to the estimated location of the bottom of the deepest plug. A logging tool including at least a collar locator and a base log sensor are lowered down the drill pipe to locate the drill pipe in its relation with the formations through which the drill pipe passes. The combination of base log/collar locator log produced on this run is correlated with the open hole reference base log to determine the exact location of the bottom of the drill pipe in the open hole. Any required adjustments to the position of the bottom of the drill pipe are made before a cement abandonment plug is placed in the open hole. A cement abandonment plug is placed in the open hole by cautiously pumping a specific mixture of water and cement down the drill pipe, displacing the drilling mud in the pipe and the bore hole. The densities of the cement mixture and the drilling mud are sufficiently similar that the cement mixture displaces the drilling mud upwardly between the drill pipe and the walls of the bore hole when the cement mixture exits the end of the drill pipe. The quantity of cement mixture pumped into the bore hole is calculated to equal the approximate volume of the required plug and is followed by exactly enough water to fill the drill pipe and completely expel the cement mixture therefrom. The drill pipe is then carefully raised until it is believed to be above the top of the wet plug.
After placement of the wet plug in the bore hole, the location of the top of the plug must be determined to be certain that the plug has the required size. Several methods for locating a cement plug in an abandoned bore hole are known. One method, called tagging, uses the drill pipe string to locate hardened or set plugs by detecting a decrease in the weight of a pipe string lowered onto a hardened plug. The disadvantage of this method is that a period of approximately eight to ten hours must elapse before the plug has hardened sufficiently to support at least part of the weight of the drill pipe, rendering this method very uneconomical. This method is also inaccurate because long drill pipe strings tend to stretch under their own weight, which makes any depth estimates, calculated by adding together the lengths of all the drill pipe sections in the hole, inaccurate by as much as several meters in deep bore holes.
A second method involves the detection of a radioactive tracer added to the whole or to parts of the cement mixture. The tracer in the cement is detected using a gamma ray detector which signals a marked increase in gamma ray readings in the proximity of the plug. The disadvantage of this method is, of course, the handling of dangerous radioactive substances and the introduction of radioactive isotopes into the environment.
Each of the methods described above establish only the location of a plug. A more recently developed method uses a densitometer, a logging tool known in the art which includes a cesium gamma ray source and a flow through sampling port. The Cesium source emits gamma radiation which is directed across the fluid in a flow through sampling port towards a gamma ray detector that measures the amount of radiation passing through the material in the sampling port. The density of a fluid in the sampling port is inversely proportional to the intensity of radiation reaching the gamma ray detector of the densitometer. Thus an increase in the density of a fluid passing through the sampling port causes a corresponding decrease in the intensity of the radiation reaching the gamma ray detector. Consequently a cement mixture, which is more dense than drilling mud, can be detected in a bore hole and an estimate of the purity of the plug can be determined by comparing the density measurement of the pure cement mixture with the density measurement of the downhole plug. Although this method provides an approximation of the purity of a wet cement plug in a bore hole, it does not provide any information about the nature of the contamination in an impure plug.
Abandonment plugs may be contaminated by a variety of substances common in well bores. Possible contaminants include lime, shale, sand, water, limestone, oil and gas. These contaminants may reduce the quality and increase the permeability of a plug by interfering with the setting of the cement, or reducing its strength by producing channels or fissures. Two disadvantages of the densitometer method of assessing the quality of a wet cement plug are that contaminants having a density equal or close to the density of the cement mixture cannot be readily detected, and the type of contamination in a wet cement plug cannot be determined. For instance it is extremely difficult to differentiate between water and oil contamination using density measurements, however their effects on plug quality are quite different. Oil, especially kerosene, is often purposefully added to a cement mixture to retard the setting of the cement and generally does not affect cement quality whereas increasing the water content of a cement mixture progressively reduces cement strength. For instance, a ratio of 40 cement/60 water produces a plug of excellent quality, while a ratio of 10 cement/90 water produces a cement mixture that will not set.
Water inflow and the introduction of drilling mud into a wet cement plug both decrease the quality of the plug. A fluid densitometer measurement cannot necessarily be used to differentiate between water and drilling mud contamination as either in the correct proportion may affect the density of a wet cement mixture to the same extent. The introduction of a certain amount of drilling mud into a wet plug may be perfectly acceptable in respect to plug quality, whereas water in a quantity to equivalently lower the density of the plug may affect the quality of the plug to the extent that it is no longer acceptable. Finally, small quantities of gas infiltration may not be detectable by a densitometer but can lead to seriously lower plug quality. The expansion forces of the gas can permeate the plug with channels, causing leaks and endangering the environment. Gas leaks are usually detected only after the well has been closed. When this occurs an expensive reabandonment process must be followed to correct the situation.
The present invention provides a novel method and apparatus for determining the location and assessing the quality of a wet cement plug, whereby a sample of the plug is captured and retrieved to the surface for examination and analysis.
The probability of poor quality cement plugs may be minimized by taking a downhole cement sample of a new plug. Taking a downhole sample and examining it for contamination, can provide information about all major parameters relating to abandonment plug quality, including: the plug border mixture density, the compressive strength, the setting time, the permeability, and the type and extent of contamination in the area where the sample is taken. Fluid density logging with a gamma ray densitometer can only determine the first and sometimes, by inference, the second of these parameters.
It is an especial advantage of the invention that it provides for the quality control of fully positioned cement abandonment plugs, thereby greatly reducing the risk of plug failure and reabandonment. Examination of a sample of a plug which is indicated by sensing tools to be of questionable density permits a determination of whether the plug will set properly, and thereby is useful in resolving disputes between a government inspector and the owner of a well regarding the quality of a positioned plug.